BAGRAM, Afghanistan (Reuters) -- Afghan and foreign troops have to respond whenever insurgents launch attacks in Afghanistan, and there is a risk civilians will also be killed, a U.S. commander has said.

Afghan and foreign forces have to decide whether to "sit back and absorb the attacks of the enemy" or try to search out those who plan, fund, and direct the attacks, Brigadier General James Holmes told reporters at the main U.S. military base in Afghanistan.

"Going after those people increases the risk of other non-combatant casualties," said Holmes, who commands air support operations for international forces in eastern Afghanistan.

The issue of civilian casualties has outraged ordinary Afghans and led to a rift between Afghan President Hamid Karzai and his Western backers. Karzai has repeatedly asked foreign troops to use more caution when carrying out raids.

The use of air strikes aimed at Taliban commanders is particularly sensitive as the results are dependent on intelligence which can sometimes prove wrong.

Civilian casualties are regrettable and must be avoided, but have to be weighed against the benefits of going after senior insurgent commanders who orchestrate attacks, Holmes said.

"Coalition forces with the Afghan government and Afghan forces must evaluate the risk, and balance it out with the gain of capturing these high-value targets," he said.

Violence in Afghanistan last year reached its worst levels since 2001, and more than 5,000 people, over 2,100 of them civilians, were killed in fighting -- a 40 percent rise from the previous year, the United Nations says.

While more than half these civilians died in insurgent attacks, a quarter were killed in air strikes by U.S. and NATO-led forces, the United Nations said.

NATO-led forces in Afghanistan say they have killed fewer civilians than the United Nations has recorded, but there has been a definite rise in the number of deaths, Holmes said.

One of the main reasons for the increase was that there had been more insurgent activity, such as roadside bombs and suicide attacks, across the whole country, which in turn increased the foreign military response, he said.

The other reason was that cases of mistaken identity sometimes led to civilian deaths, Holmes said.

Analysts say the arrival of 17,000 more U.S. troops this year, on top of the 70,000 foreign soldiers already here, will lead to a rise in violence as forces try to clear insurgents from more territory and more civilians die in the fighting.

"I believe that there will be another period of time where there will be more fighting," said Holmes, but added it was his "sincere hope" that there would not be an increase in civilian deaths as new troops were deployed